At most relatively large airports it has become common for passengers to be boarded onto and off-boarded from the waiting aircraft via a telescoping bridge. One end of the telescoping bridge is situated at the doorway of the waiting area in the terminal. When the aircraft is in position near the terminal area, the outer end of the telescoping bridge, typically supported by a wheeled support structure, is moved into position adjacent the door of the waiting aircraft.
Aircraft are equipped with an auxiliary power unit, or APU, to provide power for the aircraft while it is on the ground and the engines are not operating. The APU powers the on-board air conditioning unit and supplies power for starting its engines. However, the APU is a relatively inefficient power source so that an alternative, off-board, source for powering the on-board air conditioning unit, and also for starting up the jet engines, is desirable. Further, operating the APU adds to the relatively high level of air and noise pollution at the airport.
The use of telescoping bridge structures for passage of passengers between the aircraft and the terminal provides a vehicle by which servicing lines, such as electric cables, can be carried to the waiting aircraft. In U.S. Pat. No. 3,399,545 an expandable duct is disclosed for carrying preconditioned air to waiting aircraft. The preconditioned air is then fed directly to the aircraft so that the APU is not needed to either heat or cool the aircraft interior. The duct is preferably of a commercially available type in which a spiral wire is covered with a flexible plastic covering. The use of this longitudinally flexible material allows the duct to expand and contract in accordance with the length of the bridge. However, the air pressures which can be used with such a device are necessarily quite low, in the range of inches of water rather than pounds per square inch. Thus, such a system would be ineffective for supplying the aircraft with a supply of air power at pressures and in amounts sufficient to start the engines so that the APU must still be used during engine start-up.
One method for supplying air at pressures and in amounts sufficient to start the engines is to pull medium pressure, flexible hose from the terminal to the aircraft. However, these hoses are heavy, awkward to manipulate, and obstruct the area around the aircraft. Another method is to construct underground pressurized air conduits to the aircraft waiting area. Flexible hoses from normally covered access holes are connected to the aircraft so the APU need not be used. However, these underground systems are expensive to install and once in place restrict the areas at which aircraft can be parked. A third method is disclosed in U.S. patent application No. 4,357,860, assigned to the assignee of this application, titled Telescoping Conduit for Pressurized Air. In that application a telescoping conduit is shown supported by the telescoping bridge structure to supply the aircraft with the needed air.